How high would a stack of million dollar bills reach to pay off the national debt if they were layered one on top of the other?  That's million dollar bills, not thousands, not hundreds, but millions. 

US paper currency is .0043 inches thick.  In fifteen trillion dollars there are 15,000,000  millions.   Multiply 15,000,000 times .0043 and you get 64500 inches.  There 12 inches in a foot and 5280 feet in a mile.  12 times 5280 equals 63360 inches.  Divide 64500 by 63360 and you get 1.02. 

To pay off the 15 trillion dollar national debt in million dollar bills we would need a stack of them over a mile high.